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PESHAWAR, Pakistan (Reuters) - The release of a video showing a U.S. and a Canadian citizen held hostage by the Taliban is designed to pressure the Afghan government not to impose the death sentence on the son of a feared militant leader, a Taliban source said on Wednesday.

U.S. and Canadian officials have said they are aware of the video of Caitlan Coleman, a U.S. citizen and her Canadian husband, Joshua Boyle, who were kidnapped in 2012 while on a backpacking trip.

A State Department spokesman said on Tuesday the validity of the video was being studied but offered no further comment.

According to a senior member of the Taliban, who spoke on condition of anonymity, the timing of the release was aimed at pressuring the Afghan government not to execute Anas Haqqani, son of Jalaluddin Haqqani, founder of the feared Haqqani network.

Anas Haqqani is currently on trial in Afghanistan and faces the death sentence. Afghan media reported on Tuesday that he had probably been sentenced to death in a preliminary procedure that is not public but there has been no official confirmation or comment. The preliminary ruling would not in any case be definitive until a final verdict in the case is issued.

"The process is ongoing, we can't comment," an official at the attorney general's office said.

Taliban sources said the couple were being held by the Haqqanis, a militant Islamist group affiliated with the Taliban that is known for kidnappings and high profile attacks in cities like Kabul.

In the video, both Boyle and Coleman, who was pregnant at the time and has given birth to two children while in captivity, are shown asking their governments to pressure the Afghan government to stop executing prisoners.

In May, Afghanistan hanged six Taliban prisoners on death row following a suicide attack that killed more than 60 people in Kabul.